Composition containing a rhodamine derivative



Patented Mar. 7, 1939 UNITED STATES PATENT OFFICE COMPOSITION CONTAINING A BHODAMINE DERIVATIVE Gorton R. Fonda, Schenectady, N. 2., assignor to General Electric Company, a corporation of New York Claims.

The present invention relates broadly to a composition containing a new rhodamine derivative, specifically a rhodamine B compound in which an alkali metal is substituted for hydrogen 5 of the carboxyl group. The compositions of this invention are especiallyadapted for use in the production of light-transforming surfaces. The present application is a division of my co-pending application Serial No. 104,663, filed October 8, 1936 which latter is a continuation-in-part of a co-pending application Serial No. 60,276, flled January 22, 1936. In the parent application Serial No. 104,663, claims are made, including both method and article features, to light-transforming screens or reflectors which may be produced with the compositions claimed in this application.

In my co-pending application Serial No. 60,276 I have disclosed and claimed a fluorescent screen or reflector of the type used for correcting the spectre of light sources which are deficient in one or more spectral colors and the combination of such screens and light sources. In accordance with that invention a fluorescent screen having a much longer life than attainable with similar screens theretofore known and which is also highly eflicient in converting impinging radiant energy into fluorescent light of a complementary color is provided by employing a fluorescent coating having a thickness in excess of that required to transform completely the impinging radiations, but disposed in such a discontinuous fashion that the flnal blending of transformed and untransformed light will afford a substantially proper spectral distribution. In my co-pending application Serial No. 60,276, I disclosed but did not claim a derivative of rhodamine B formed by substituting the alkali metal potassium for hydrogen of the carboxyl group of rhodamine B, and the use of this compound in solution form as a fluorescent coating material for application to a surface to form a light-transforming screen or reflector. In the following specification I shall describe with greater particularity alkali metal derivatives of rhodamine B, and how they may be made and used.

The present invention is based upon the observation, not heretofore known and which was not obvious from the literature describing the properties of rhodamine B, that a rhodamine dye in which an alkali metal has been substituted for hydrogen of the carboxyl group of rhodamine B yields a fluorescence which is more permanent under exposure to light than rhodamine B.

It has heretofore been known, that rhodamine and derivatives thereof may be incorporated in compositions that may be used for transforming light rays and, also, for adding light waves of definite character to light from any source. Such compositions are disclosed in the British Patent 5 16,271 of 1911 to Peter Cooper Hewitt, and in U. S. Patents 1,150,118, 1,888,655 and 1,188,777, also to Hewitt. I make no broad claim to the use of rhodamine or any derivative thereof in combination with other ingredients for such uses as men- 10 tioned above, but I do claim as new and novel an alkalimetal salt of rhodamine B and its use in particular compositions for making light-transforming screens or reflectors having such improved permanence under exposure to light as to 15 provide a screen that is, in its practical effect, different in kind from light-transforming screens heretofore known to the art.

In the literature on thep fluorescence of rhodamine, the form thereof commonly referred to is 20 Schultz No. 573, which is a chloride of a rhodamine B base. It has the empirical formula CzaI-InNzOaCl and the constitutional formula c1 4 25 (0.1mm moan).

The term- "rhodamine B as used, generally,

herein refers to the form of rhodamine B in common use as a dye, that is in the chloride form. Such form of rhodamine B also is known as rho- 35 damine B hydrochloride. The rhodamine alkali metal salt derivative embodying my invention has the empirical formula CzaHaoNzOaClR and the constitutional formula (Cn' OCOOR tion of the rhodamine B salts of the other alkali metals, viz. lithium, sodium rubidium and caesium.

Five grams of rhodamlne B are dissolved in a solvent or mixture of solvents in which the potassium salt of rhodamlne B is substantially more insoluble than rhodamlne B. Examples of suitable solvents are acetone, ethyl alcohol, or mixtures thereof, or mixtures of either or both with water. Advantageously a 50:50 mixture of water and ethyl alcohol of about 95 per cent strength is employed. To such a solution is added a solution of a similar alcohol-water solvent containing an equi-molecular amount, and advantageously slightly more than the equi-molecular amount of potassium hydroxide requiredto form the potassium salt of rhodamine B, for example. 0.6! gram of potassium hydroxide. This mixed solution is evaporated until crystals appear. The crystals are separated from the mother liquor by suitable means, for instance, by vacuum filtration. The separated crystals are dissolved in a fresh mixture of alcohol and water and evaporated again until crystals appear. The purified crystals are separated from the solution by vacuum filtration or other convenient means and then are dried, for example, over phosphorous pentoxide.

Compositions for coating screens or other backing or supporting members may be made as hereinafter described, with solid alkali metal salt of rhodamlne B prepared in the manner described and, for Pm of illustration, the potassium salt of rhodamlne B is specified. A suitable amount, for instance, 2.0! grams of the potassium salt of rhodamlne B, is dissolved in one liter of an appropriate solvent such as ethyl alcohol. For use in a coating composition, the alcohol solution of the potassium salt of rhodamlne B is diluted to a ten-fold volume by adding a solution of cellulose acetate. advantageously a solution of cellulose acetate in acetone and containing, for example, about 30 per cent cellulose aceingly less than that shown by rhodamlne B.

Thus, a thin film composed of rhodamlne B and a binder of cellulose acetate. when exposed to a quartz mercury-vapor lamp at 'a distance of 2% inches (4000 foot candles) showed a loss of about '15 per cent of its original fluorescence after 200 hours exposure .and almost a complete loss of fluorescence after 400 hours exposure. A similar film composed of the potassium salt of rhodamlne B and cellulose acetate, when exposed in a like manner, showed a loss of only 10 per cent of its original fluorescence after 1500 hours exposure.

Another example of a method of preparing an alkali metal salt of rhodamlne B and a fluorescent coating composition containing such a salt, and again for illustrative purposes the potassium salt is mentioned, is as follows:

Equi-moiecular solutions of rhodamlne B and potassium hydroxide are prepared by separately dissolving 1.915 grams of rhodamlne B per liter of a suitable solvent such as acetone or ethyl alcohol and 0.220 gram of potassium hydroxide per liter of the same solvent as used with the rhodamlne B. These solutions are mixed and then diluted to a 5-fold volume by adding thereto a solution of cellulose acetate in a solvent which is miscible with the solvent used in making the mixed solution of rhodamlne B and potassium hydroxide, advantageously a solution of cellulose acetate in acetone and containing, for example, about 30 per cent cellulose acetate. As in the previous illustration, one liter of the composition prepared as described contains a weight of the P tassium salt of rhodamlne B which is 0.0004 times its molecular weight.

l'br convenience in applying to light-reflecting surfaces a fluorescent coating composition made in such ways as hereinbefore set forth, that is 1 to say, in order to improve its brushing properties,

various high-boiling diluents of the kind used in ordinary varnishes-may be added thereto. Examples of such diluents are ethyl lactate, monomethyl ether of ethylene glycol and the acetate of the mono-ethyl ether of diethylene glycol ("Carbito acetate). Advantageously, I have added, per cent by volume of ethyl lactate to fluorescent coating compositions of the kinds above mentioned, to obtain a composition of improved brushing properties. Larger or smaller amounts of such diluents may be employed in order to improve the spreading and brushing properties of the composition. The composition may be applied to a surface without further dilution or, in certain cases, for example. when it is desired to form a very thin fluorescent film, the coating composition may be further thinned with a suitable solvent such as acetone or alcohol.

The intensity of fluorescence of a rhodamlne dye is dependent not only upon the particular rhodamlne dye but also upon the particular solid solvent or binder used in its application. I have found that greatest efllciency and life of a fiuorescent film comprising the hereinbefore mentioned class of organic compounds is realized when such solid solvents or bindersas cellulose acetate or Formvar" are used. Formvar is the trade name of a classof compounds produced, generally speaking, by reacting a partially or completely hydrolyzed polymerized vinyl ester with an aldehyde. v The preparation of such compounds is set forth, for example, in U. 8. Patent No. 2,036,092, to Morrison et al. Binders other than cellulose acetate and fFormvar" may be used, and may be of either organic or inorganic composition. Any suitable liquid solvent may be employed in applying the alkali metal salt of rhodamlne B andbinder therefor to the reflecting surface to be coated. Supporting members for the composition may comprise, for example, such substances as metals, for instance, alumi-' num; nickel, chromium, steel, etc., the surfaces of which are opaque and such as to produce diffused reflection; and such other materials, having the same opaque and diffusingcharacteristics described with reference to metals, as special glasses, porcelain, enameled surfaces, paper, cardboard, or silk, rayon, linen, cotton, or other textile fabric. I

In preparing and using a coating composition containing an alkali metal salt of rhodamlne B, it is important, in order to obtain maximum resistance .of a film thereof to fading when exship between said salt and binder as they are present in the dried film. This relationship can be most easily expressed by specifying the concentration of the alkali metal salt of rhodamlne B in the liquid solution containing the preferred binder. Although I have found, taking the potassium salt of rhodamine B as a specific examposed to light. that there be a definite relationple, that best results are obtained when said salt is applied to a supporting member in the form of a solution containing, as a maximum, about 0.2 gram thereof per liter of solution, such preferred concentration is not essential in eiIectively utilizinrthe, rhodamine salt of this invention for fluorescent purposes. The concentration of the salt may be substantially diminished, for example, to as low as about 0.02 gram of the salt per liter of solution, and for the same concentration of cellulose acetate solution (or I solution of other binder), without marked effect upon the intensity of fluorescence and life of the dried film. In other words, for the same concentration of binder, advantageously I may use, in making a light-transforming screen or reflector, a solution containing a weight of alkali metal salt of rhodamine B per liter of solution that is about 0.00004 to 0.0004 times its molecular weight. As pointed out in my co-pending application, Serial No. 60,276, filed January 22, 1936, a considerable increase in the efliciency of a fluorescent film is obtained if stripes of the film are applied with an optimum thickness, which, for a concentration such as indicated above as preferable, should be between about 2 and 14 mils.

Alkali metal salts of rhodamine Bproduced in accordance with this invention may be applied to a supporting or a backing member to form either a continuous or a discontinuous surface. They may be applied so as to form vertical, horizontal or other stripes, or the surface may be completely coated with the composition. They may be applied to a screen or reflector used in connection with a metal-vapor lamp, for instance, a mercury-vapor lamp, and in such use improve the quality of the illumination provided by such sources by producing the complementary color red, of which the light given of! by the ordinary mercury-vapor lamp is almost entirely devoid. Alkali metal salts of rhodamine B, together with a suitable binder, may be applied to a lighttransforming screen adapted both to transmit and to reflect light in the manner described with greater particularity in my co-pending application Serial No. 60,276, filed January 22, 1936, and hereinbefore mentioned- They may also be used in making a light-transforming screen comprising a support and a composite coating thereon obtained by alternately applying to said support, first a solution containing the alkali metal salt of rhodamine B, a binder therefor, such as cellulose acetate or Formvar", and a liquid solvent; and then a solution of' the binder and liquid solvent therefor. Although the coating compositions may be applied in either order, preferably the solution containing the rhodamine salt and binder is applied first and the solution of the binder is applied as the final coating.

The respective coatings are dried after each application by any suitable means, for example, under artificial heat or by exposure to the air. This method of making a light-transforming screen and the advantages of this procedure, viz., a longer screen life, are set forth in more detail in my co-pending application Serial No. 60,276 previously mentioned.

As an alternative procedure to that hereinbefore described for making a light-transforming screen and wherein a permanent supporting or backing member is employed, the coating composition comprising the alkali metal salt of rhodamine B and a binder therefor may be applied to a suitable temporary support, dried, and then stripped from the temporary support and used independently of any other support as a lighttransforming or fluorescent screen.

what I claim as new and desire to secure. by Letters Patent of the United States is:

1. A composition for making a light-transforming surface comprising alkali metal salt of rhodamine B hydrochloride and a binder therefor.

2. A composition for making a light-transforming surface comprising alkali metal salt of rhodamine B hydrochloride and a product of reacting hydrolyzed polymerized vinyl ester with an aldehyde.

3. A composition for making a light-transforming surface comprising the potassium salt of rhodamine B hydrochloride and cellulose acetate.

4. A liquid coating composition comprising alkali metal salt of rhodamine B hydrochloride, a binder therefor, and liquid solvent for said salt and binder.

5. A liquid coating composition comprising the potassium salt of rhodamine B hydrochloride.

cellulose acetate, and acetone.

. GORTON R. FONDA. 

